mount: Support "nosymfollow" in new mount api

Commit dab741e ("Add a "nosymfollow" mount option.") added support
for the "nosymfollow" mount option allowing to block following symlinks
when resolving paths. The mount option so far was only available in the
old mount api. Make it available in the new mount api as well. Bonus is
that it can be applied to a whole subtree not just a single mount.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Mattias Nissler <mnissler@chromium.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ross Zwisler <zwisler@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Author: Christian Brauner

fs/namespace.c

@@ -3464,9 +3464,10 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 	return ret;
 }
 
-#define FSMOUNT_VALID_FLAGS \
-	(MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV | \
-	 MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME)
+#define FSMOUNT_VALID_FLAGS                                                    \
+	(MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV |            \
+	 MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME |       \
+	 MOUNT_ATTR_NOSYMFOLLOW)
 
 #define MOUNT_SETATTR_VALID_FLAGS (FSMOUNT_VALID_FLAGS | MOUNT_ATTR_IDMAP)
 
@@ -3487,6 +3488,8 @@ static unsigned int attr_flags_to_mnt_flags(u64 attr_flags)
 		mnt_flags |= MNT_NOEXEC;
 	if (attr_flags & MOUNT_ATTR_NODIRATIME)
 		mnt_flags |= MNT_NODIRATIME;
+	if (attr_flags & MOUNT_ATTR_NOSYMFOLLOW)
+		mnt_flags |= MNT_NOSYMFOLLOW;
 
 	return mnt_flags;
 }

include/uapi/linux/mount.h

@@ -120,6 +120,7 @@ enum fsconfig_command {
 #define MOUNT_ATTR_STRICTATIME	0x00000020 /* - Always perform atime updates */
 #define MOUNT_ATTR_NODIRATIME	0x00000080 /* Do not update directory access times */
 #define MOUNT_ATTR_IDMAP	0x00100000 /* Idmap mount to @userns_fd in struct mount_attr. */
+#define MOUNT_ATTR_NOSYMFOLLOW	0x00200000 /* Do not follow symlinks */
 
 /*
  * mount_setattr()